A CMP (chemical mechanical polishing) process is the final process in the fabrication of silicon wafers to lower the microroughness of wafer surface to planarize the wafer surface, and remove physical surface defects such as microscratches and pit marks. After the wafers are polished by the CMP process, they have a specular surface with low surface defects. Microscratches remaining on the wafer surface are defects that cause a problem during a photolithography process for implementing a circuit. Accordingly, it is important to reduce the number of the microscratches during the CMP process.
The CMP process for polishing wafers is normally carried out in two or more steps. The primary polishing step is a polishing step requiring a high polishing speed to remove deep scratches on the wafer surfaces. A secondary (final) polishing step is required to accomplish a specular surface by removing microscratches remaining after the primary polishing step and lowering the microroughness of the surface to the level of several Å.
In addition to a polisher and deionized water, the following two elements are required for polishing of the wafer surface: a soft or hard urethane polishing pad, and a silica slurry as a polishing solution. The wafer surface polishing process is explained based on a chemical mechanical polishing reaction. The polishing pad effects the mechanical polishing, while the polishing solution (slurry) effects the chemical polishing. The polishing solution also plays a role in assisting the mechanical polishing by the polishing pad. Demands for large diameter wafers and high quality wafers require improved performance of the polishing pad and the slurry. Particularly, since the slurry is considered to be a final regulator for high quality wafers, slurry products having various physicochemical properties are now commercially available in the market and studies on them are being actively undertaken.
The slurry used in the CMP process generally consists of an abrasive, a base as a pH-adjusting agent and deionized water. In addition, the slurry may further comprise an organic or inorganic additive depending on special intended polishing quality.
Silica is mainly used as the abrasive, and potassium hydroxide, sodium hydroxide, or aqueous ammonia is commonly used as the pH-adjusting agent. Examples of the additives for increasing polishing speed, cleanness of the surface to be polished and dispersibility of the abrasive include nonionic surfactants, polishing speed enhancers such as amines, etc.
It is common to employ different slurries depending on the characteristics of each CMP process. The primary polishing slurry and the second polishing slurry have different physical properties in order to increase the characteristics of each step. The first polishing slurry for increasing the polishing speed comprises an abrasive having a large particle size (80˜120 nm) and a high concentration (2˜30 wt %) and is adjusted to a pH of 11˜12, thereby improving mechanical and chemical polishing force. On the other hand, the second polishing slurry for removing surface defects instead of increasing the polishing speed comprises an abrasive having a small particle size (10˜80 nm) and a low concentration (0.2˜10 wt %), thereby decreasing the mechanical polishing effect but increasing the chemical polishing effect. Various functional additives may further be added to the secondary polishing slurry for realizing a smooth specular surface.
Trednnick et al. (U.S. Pat. No. 3,715,842) discloses a method for producing a final polishing slurry comprising the steps of dispersing silica particles having a particle size of less than 100 nm in water, adding 0.05% by weight or more of ammonia to the dispersion to adjust the pH to 7 or higher, and adding 0.05˜2.5% by weight of hydroxymethylcellulose (HMC), hydroxyethylcellulose (HEC) and hydroxypropylcellulose (HPC) thereto, thereby preventing the precipitation of the silica particles and reducing scratches.
Payne et al. U.S. Pat. Nos. 4,169,337, 4,462,188 and 4,588,421) describes compositions for increasing the polishing speed which comprises silica particles having a particle size of 4˜100 nm, and 2˜4% by weight of an amine selected from aminoethanolamine and ethylenediamine or 2˜4% by weight of a quaternary ammonium salt selected from tetramethylammonium chloride or tetramethylammonium hydroxide (TMAH).
Sasaki et al. (U.S. Pat. No. 5,352,277) suggests a polishing slurry comprising colloidal silica, a water-soluble polymer and a water-soluble salt. The colloidal silica comprises 20 to 50% by weight of silica with a particle size of 5˜500 nm, the water-soluble polymer is present at a concentration of about 100 ppm, and the water-soluble salt is present at a concentration of about 20˜100 ppm and consists of a cation selected from Na, K and NH4 and an anion selected from Cl, F, NO3 and ClO4. This slurry can realize a soft surface having a low surface roughness of less than 5 nm.
Lonckid et al. (WO 96/38262) suggests a slurry composition for final polishing of silicon wafers which comprises 0.2˜0.5% by weight of silica having a particle size of several hundred nanometers or less, 0.01˜0.1% by weight of an amine, and 0.02˜0.05% by weight of PVA (polyvinyl alcohol). The amine is used to adjust the pH of the slurry at pH 8˜11. It was observed that when a 150 mm wafer was polished using the slurry, the haze was 0.06 ppm and the LPDs (Line Point Defects) having a size of 0.1˜0.3 microns were less than 90. However, nothing was taught about the other defects in this international publication.
Japanese Patent Application No. 2000-6327 of Inoue et al. discloses a composition for a secondary or a final polishing. In the composition, silica having a particle size of 20˜300 nm is used as an abrasive and 0.001˜0.3% by weight of TMAH is used as a base. In addition, this patent application discloses the addition of hydroxyethylcellulose (HEC) having a molecular weight of 1,300,000 or more for improving the surface hydrophilicity of the polished wafers. However, the composition obtained has not been satisfactory compared as common polishing slurries.
Recently, there is a need for high quality slurries in view of microroughness and pitted microscratches, as well as surface defects such as haze and LPDs (Low Point Defects). In particular, since pitted microscratches tend to arise due to unbalanced chemical polishing by a polishing slurry, considerable care is required during mixing the slurry.
The present inventors have conducted an intensive study to find out a slurry composition capable of increasing the dispersion stability of silica as an abrasive to improve the polishing quality on pitted microscratches and reducing the amount of silica particles in the composition to lower the manufacturing cost. As a result, the inventors have found a slurry composition with low concentration of silica and high dispersibility.